1. Field of the Disclosure
The disclosure is related generally to the field of making resistivity induction measurements using well logging instruments for the purpose of determining the properties of earth formations. More specifically, the disclosure is related to a method for improving the performance and simplifying the mechanical requirements for multi-component induction logging tools and propagation resistivity logging tools.
2. Background of the Art
Electromagnetic induction and wave propagation logging tools are commonly used for determination of electrical properties of formations surrounding a borehole. These logging tools give measurements of apparent resistivity (or conductivity) of the formation that, when properly interpreted, are diagnostic of the petrophysical properties of the formation and the fluids therein.
Downhole induction instruments employ multiple sensing coils which could operate in “stand-alone” or “array” modes. By a “stand-alone” mode we mean having individual pairs of these devices with one coil serving as a transmitter and another as a receiver to sense the secondary magnetic field induced by eddy currents appearing due to formation interaction with the transmitter magnetic field. If the assembly has three or more coils participating in measurement, it would acquire an additional property such as ability to spatially focus magnetic field response. This could be done by either re-arranging eddy current flow path in the formation or by complex weighting the induced magnetic field responses in receivers prior combining them.
However, there is a significant problem in such instruments having multiple arrays. The issue lies in the conventional principle of the tool design and, in particular, the necessity for these arrays to operate in the same frequency range. This typically leads to a sequential (instead of a simultaneous) mode of measurements and often may result in lower than possible accuracy of the acquired data.
The decrease in accuracy results due to both tool movement in the well (which results in recording information from different locations), and decreasing an effective signal acquisition time. There are also restrictions associated with measurement errors due to unavoidable magnetic coupling inside the tool. Yet another limiting factor could be the formation response itself.
Baker Atlas and Shell International E&P jointly developed a multicomponent induction logging tool, 3DEX® to measure the electrical anisotropy of these sequences. This logging tool and its use is described in U.S. Pat. No. 6,147,496 to Strack et al. The instrument comprises three mutually orthogonal transmitter-receiver configurations that provide all necessary data to compute horizontal and vertical resistivities of the formation. These resistivities may then be used in an integrated petrophysical analysis to provide an improved estimate of the laminar sand resistivity and corresponding net oil-in-place. The tool was originally developed for wireline applications, but the principles have been extended to measurement-while-drilling (MWD) applications. We discuss next, as an example, the use of such a device in wireline applications.
Referring now to FIG. 1, an electromagnetic induction well logging instrument 10 is shown disposed in a wellbore 2 drilled through earth formations. The earth formations are shown generally at 4. The instrument 10 can be lowered into and withdrawn from the wellbore 2 by means of an armored electrical cable 6 or similar conveyance known in the art. The instrument 10 can be assembled from three subsections: an auxiliary electronics unit 14 disposed at one end of the instrument 10; a coil mandrel unit 8 attached to the auxiliary electronics unit 14; and a receiver/signal processing/telemetry electronics unit 12 attached to the other end of the coil mandrel unit 8, this unit 12 typically being attached to the cable 6.
The coil mandrel unit 8 includes induction transmitter and receiver coils, as will be further explained, for inducing electromagnetic fields in the earth formations 4 and for receiving voltage signals induced by eddy currents flowing in the earth formations 4 as a result of the electromagnetic fields induced therein.
The auxiliary electronics unit 14 can include a signal generator and power amplifiers (not shown) to cause alternating currents of selected frequencies to flow through transmitter coils in the coil mandrel unit 8.
The receiver/signal processing/telemetry electronics unit 12 can include receiver circuits (not shown) for detecting voltages induced in receiver coils in the coil mandrel unit 8, and circuits for processing these received voltages (not shown) into signals representative of the conductivities of various layers, shown as 4A through 4F of the earth formations 4. As a matter of convenience the receiver/signal processing/telemetry electronics unit 12 can include signal telemetry to transmit the conductivity-related signals to the earth's surface along the cable 6 for further processing, or alternatively can store the conductivity related signals in an appropriate recording device (not shown) for processing after the instrument 10 is withdrawn from the wellbore 2.
Referring to FIG. 2, the configuration of transmitter and receiver coils in an embodiment of the 3DEX® induction logging instrument of Baker Hughes is shown. Three orthogonal transmitters 101, 103 and 105 that are referred to as the Tx, Tz, and Ty transmitters are shown (the z-axis is the longitudinal axis of the tool). Corresponding to the transmitters 101, 103 and 105 are associated receivers 107, 109 and 111, referred to as the Rx, Rz, and Ry receivers, for measuring the corresponding magnetic fields. In one mode of operation of the tool, the Hxx, Hyy, Hzz, Hxy, and Hxz components are measured, though other components may also be used. The arrangement of coils shown in FIG. 2 is not intended to be a limitation, and there are devices in which the different coils are collocated. U.S. patent application Ser. No. 11/858,717 of Signorelli having the same assignee as the present disclosure and the contents of which are incorporated herein by reference teaches the use of collocated antennas for multicomponent resistivity tools.
In instruments with collocated coils, there always exists magnetic coupling between mutually-orthogonal coils assembled in the same place on the mandrel; however, even infinitively accurate design would not free the instrument from interferences. These interferences become dependent on multiple logging factors such as tool eccentricity in the well, borehole wall rugosity and even mud homogeneity.
Having arrays with the leads running along the mandrel from coil terminals to the respective electronic amplifiers, as well as amplifiers themselves with finite input impedances, introduces a load for the coils and thus results in a parasitic current-conduction in the coil itself and displacement one in the associated cables. The currents, in turn, produce magnetic fields which couples in neighboring arrays and thus create errors in form of induced voltages which contribution could not be removed by the following signal processing as these value as phase-synchronized with signals of interest.
The instrument has to produce phase-discriminated measurements of both main (xx, yy or zz) magnetic field components as well as respective cross-components (namely, xy, xz, etc.). However, if all transmitters run simultaneously, it is not possible to separate different components in the receiver, i.e., if receiver X is considered, the voltages induced by magnetic fields from x-, y- and z-directional transmitters could not be distinguished.
One approach may be to have mutually-orthogonal transmitters operating at different frequencies simultaneously. This is a very difficult technical issue, and mutual interference of coherent frequencies remains.
A similar problem is encountered in multiple propagation resistivity tools. In this propagation instrument, two transmitters operate sequentially with respect to any given receiver. There are multiple reasons for this type of measurements; however, the main issue remains the same: if both transmitters run simultaneously there are no prior art devices for separating the respective formation phase-discriminated phase responses induced in the receivers. FIGS. 3A, 3B, and 3C are simplified schematic depictions of several alternative possible antenna configurations which may be utilized in an MPR device implemented as a logging-while-drilling apparatus. See U.S. Pat. No. 5,869,968 to Brooks et al., having the same assignee as the present disclosure. The embodiment of FIG. 3A is a dual transmitter, dual receiver antenna configuration which includes upper receiving antenna 351, lower receiving antenna 357, and closely-spaced transmitting antennas 353, 355 which are positioned intermediate receiving antennas 351, 357. The transmitting and receiving antennas are substantially symmetrically positioned about a center line which is located intermediate transmitting antennas 353, 355.
FIG. 3B is a simplified schematic depiction of another embodiment of the closely-spaced transmitter embodiment of the present disclosure. As is shown, transmitting antennas 363, 365 are positioned at a medial location of measurement tubular. Receiving antennas 359, 361 are located at an upper distal portion of measurement tubular 25. Receiving antennas 367, 369 are located at a lower distal portion of measurement tubular.
FIG. 3C is a simplified schematic representation of yet another embodiment of an MPR device. In this particular embodiment, a single receiving antenna 371 is located at an upper distal portion of measurement tubular. Transmitting antennas 373, 375 are located at a medial portion of measurement tubular, and are closely-spaced to one another, as compared to the spacing between either of the transmitting antennas and the single receiving antenna. Alternatively, receiving antenna 371 could be located at a lower distal portion of measurement tubular 25.
There are several hardware and/or calibration solutions to the problems of stray coupling. See, for example, U.S. Pat. No. 6,586,939 to Fanini et al., U.S. Pat. No. 7,190,169 to Fanini et al., and U.S. patent application Ser. No. 11/627,172 of Forgang et al. This disclosure is directed towards a method and apparatus which effectively avoids the coupling problem.